Thermionic device



Jan. 13, 1959 R. A. MULLER THERMIONIC DEVICE Filed Oct. l0, 1955 FIG.

Il vVElvToR. RUDOLF AMUELLER FIGA ,a 7' TURA/Er to travel in a generally helical THERMIoNIc DEVICE Rudolf A. Mller, Munich, Germany, assignor to the United States of America as represented by the Secretary of the Army Application October 10, 1955, Serial No. 539,707 z claims. (61.*313-157) (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without'the payment of any royalty thereon.

This invention relates to electronic discharge devices and .particularly to such devices having a cathode and an anode wherein the harmful effects of back bombardment upon the cathode are substantially eliminated.

The short life of cathodes such as those used in conventional microwave generators is due in large part to the fact that electrons emitted therefrom and failing to reach the anode return to the cathode. The impact of the returning electrons tends to destroy the emmissive surfaces of the cathode and also tends to cause unsatisfactory operation of the device due to substantial components of 4noise which enter the output thereof. The structure of the cathode and anode in such conventional devices causes electrons leaving the cathode to travel in a at plane outward at right angles to the cathode surface as they move toward the anode. Some of these electrons fail to reach the anode and return tothe cathode in substantially the same zonal area of the cathode as that from which they have left it.

The invention in effect separates the returning electrons Vfrom the primary electrons which reach the anode and cause the returning electrons to follow a path which takes them out of the zone of high emission on the cathode. The returning electrons in striking the inactive areas of the cathode do not` destroy the emissive property of the active area.

Briefly stated, the invention consists in providing a cathode and anode combination in which the interaction space therebetween is such that the distance between the cathode and anode progressively changes along the axis of the elements. When a voltage is applied between these elements, a special type of electric iield is created which imparts to electrons entering the field a component of motion axially of the device. This component together with the outward force on the electrons due to the electric eld and the rotative force resulting from a magnetic field applied axially along the cathode causes electrons path extending along the said axis. All of the electrons leaving the cathode follow this helical path; the primary electrons reaching the anode provide the normal output of the device, while those electrons which fail to reach the anode return thereto at a point somewhat remote from their point of departure.

The invention utilizes this special excursion of the returning electrons to prevent destruction of the emissive area of the cathode and also to prevent the generation of noise components associated with this back bombardment phenomenon. To do this the cathode is coated or activated along an annular zone including only a portion of the cathode surface. In this manner the returning electrons are caused to engage the cathode at points remote from the emissive zone thereon. Thus the ill effects of the bombardment are eliminated.

United States Patent() It is a primary object of the invention to provide a cathode of maximumruggedness and long life.'

It is a further object of the invention to provide a cathode of high output capacity and one which creates a minimum of noise in the output of the device in which it functions. 1

A still further object of the invention is to provide a highly eliicient cathode in which the harmful effects of back bombardment are substantially eliminated.

Other objects and features ofthe invention will .more fully appear from the following description and will be particularly pointed out in the claims.

To provide a better understanding of theinvention a particular embodiment thereof will be described and illustrated in the drawings in which: p l

Fig. l is a cross-section of a preferred 4form of the invention as applied to a magnetron, the section being taken in a plane on the axis of the device.

lFig. 2 is a cross-section on line 2 2, Fig. 1.

Fig. 3 is a cross-sectional view similar to Fig. l illustra'ting a different form of the invention.

Fig. 4 is a diagrammatic view illustrating the path of an electron which has left the cathode and returned thereto.

The invention will be described in connection with its application to a magnetron although it will be understood that the invention may be applied to other electronic devices. The magnetron illustrated is of the construction which includes the ring type anode 5 having inwardly projecting segments 6 whichembrace resonator cavities 7 therebetween. The anode is provided with a conventional output coupling loop 8 from which power is drawn from the device.

The cathode 9 is of special construction and in one specic form `of the invention may 'be of conical conformation. The cathode may, however, have any contour which presents a surface or surfaces disposed symmetrically about an axis.` The cathode is positioned on the axis of the yanode l5 equally spaced from the ends of the segments 6. The cathode may be made of nickel or other suitable metal and may be solid or hollow. A suitable arrangement is to provide a chamber within the cathode to receive a conventional electric heating coil 10 imbedded in ceramic material filling the chamber. The heater provides suitable temperature for the required electron emission. Y

The cathode has an annular zone I11 which is rendered highly emissive or is coated wit-h emissive material. The zone is relatively narrow with respect to the length of the cathode and therefore provides a radial Abeam of electrons of generally disc-like conformation extending from the cathode to the anode.

Various types of emissive zones may be provided, for example, in placeof the zone 11 upon the cathode surface. The cathode may have a groove for-med therein within which a copious supply of active material may be held thus extending lthe life of the device. The magnetron is provided with a suitable voltage gradient between its cathode and anode and in addition a magnetic field is applied axially of the cathode extending between the poles 12 and 13 of a suitable magnet.

It will be noted that the space 'between the cathode surface and the ends of `the anode segments 6 'tapers in width, which in effect distorts the electric field so that the -ux thereof follow cu-rved paths. Electrons emitted from the cathode start their ight in a plane perpendicular to the cathode surface 14 and during their excursion to the anode are caused to follow a generally helical path striking the anode in a plane perpendicular to the faces of the anode segments '6. This distortion of the field has no serious effect upon the major output of the device but;

Y moving causes outward movement` :thereof toward Hthe yanode and the magnetic eld causes the electrons to travel in a circular path labout theaxis of the cathode. ,The combined vectors VTof "these forces cause the electrons tofollow ia helical path shown diagrammatically in Fig. 4. Thetota-lexcursionr'outward and then inward along this Ygeneral-ly Vhelical pathils causes Ythe electrons to strike4 the cathodesalong a zone at a point `of contact 16 shown in Fig. 4"Which` is 0remote from the zone 11 of active emission.y VThe emissi-ve zone-11 is therefore protected against back vbombardment of these electrons. Without such protectionthe limpact of returning electrons dislodges particles from the emissive area which may .become ionizedin 1 the interaction space. When1such ionization occursundesirable noise :modulation of the outputtakes place. The invention, therefore, eliminates vor greatly reduces output noise components.

A different form of the invention is shown in Fig. 3

wherein the cathode 17 is cylindrical instead of conical as in Fig.. 1. lIn ythis construction the faces of the anode segments 1S are disposed at an angle to the axis yof the cathode. This construction provides the same tapered interaction space asA that provided in the form of the device shown in Fig. 1. The. device shown in Fig. 3 functionsnin the samemanner as `the device shown in Fig. l as will become evident from the description of the operation -of the device shown in Fig. l. Thus it is clear that the inventiondoes :notvreside solely in the contour Aof the cathode but does reside in its contour with respect tothe contour of the interacting faces of the segments of the anode'S. The tapering, conformation of the interaction space is vthe feature .which causes lthe electrons to follow a generally helical pathas compared to the lspiral path followed by electrons traversing the interaction space in conventional 'magnetrons or similar devices.

Any suitable means may he employed to support the cathodes 11 or y17. As show-n in the drawings the lead wires 19 for the heating coil 10, are'secured in insulating bushings 2t)r which in turn arevsecured in the wall `of the anode 5.

Although t-he conventional relative position of the cath-- ode within the anode has been shown and described, it is within the scope of the `invention to lreverse -this relationship and position the anode within the cathode. =In this construction the opposingfaces of the two elements would be shaped to present la tapered interaction spacey as in the other forms of -the invention.

What is claimed is:

1..An electronic. discharge device .comprising a cath ode whose contour presents a surface of revolution, an anode surrounding said cathode, the opposing'surface of said cathode and :anode bounding Yan interaction space therebetween in which the distance from the cathode to the anode progressivelyiincreases along the cathode axis, an annular zone of high emission on said cathode covering only a portion ofthe area thereof said zone being spaced from the plane of greatest interaction space, the remaining `area of the :cathode: embraced .by the anode having `a low-emission characteristic, means to -create an electric eld between` cathode and anodeand a magnet whose poles embrace .said interaction space.

2. -An electroniodischarge device: comprising a cathode having an-external; areadisposed symmetrically' about an axis, a circular anode surrounding` the cathodepresenting inwardly `projecting segments having spaces therebetween, the opposing surfaces of `said anode segments Yand said cath-ode bounding an interaction space `therebetween in which the distance from cathode to anode progressively increases along said-axis, an annular zone `of high emission .on the said cathode including a portion only of the surface thereofysaidzone being spaced from the plane of greatest interactionspaee, the remaining area ofI said cathode-embraced bythe anode'having a -low emission characteristic, means tocreate anl electriceldlbetween the; cathode and'anode `and `a magnetv having itspofles embracing said interaction space.

References Cited in the Yfile of this patent UNlTED STATES PATENTS 1,695,746 Stark Dec. 18, '1928 1,957,327 `llser May 1, 1934 42,048,224 Snow "7 July 21, 1936 2,079,248 Fritz May 4, 1937 2,412,372l y Usselman Dec. `10, 1946 FOREIGN PATENTS 318,624 'Germany Jan. '31,1920 

